Organic & Biomolecular Chemistry Blog

Synthetic methodologies to control the selectivity and specificity of catalytic reactions remain the subject of intense research due to the importance of chiral compounds in pharmaceuticals, agrochemicals and functional materials. Homogenous catalysis takes advantage of weak interactions between the substrate, catalyst and groups distal to the active site to impart selectivity. Such features are ubiquitous in enzymes which display exceptionally high levels of stereochemical control and activity. In order to exploit the advanced capabilities of such enzymes for selective catalytic transformations, researchers have linked transition metal catalysts with different biological scaffolds–proteins, peptides, DNA–to create artificial metalloenzymes.

Since their inception in the late 1970’s, artificial metalloenzymes have emerged as a vibrant area of research with numerous examples of a variety of catalytic transformations reported alongside creative methods for incorporating the transition metal complex into the biomolecular scaffold. Key to their design is the ‘second coordination sphere’ provided by the biological scaffold where supramolecular interactions within the active site contribute to the rate and enantioselectivity of the system.

Prof. Thomas Ward and Prof. Oliver Wenger of the University of Basel have recently reported a novel biotin-streptavidin system equipped with an anchored photosensitizer capable of undergoing electron transfers between the biotinylated electron donor and ruthenium(II)-labeled streptavidin.

In the past, studies of the luminescent properties of biotinylated d⁶ metal complexes, common in photoredox catalysis, have been carried out for the purposes of cell imaging and ruthenium and rhenium complexes have been employed in the elucidation of electron tunneling pathways of various proteins. The application of phototriggered electron transfers to ruthenium photosensitizers, up until this point, had not yet been realized and this recent discovery demonstrates a significant advancement within this field.

This new biotin-streptavidin artificial metalloenzyme contains a ruthenium catalyst and photosensitizers embedded within the biotin binding pocket of streptavidin through covalent interactions with non-native cysteine residues. Their performance in various electron transfer studies demonstrates their potential to behave as advanced photoredox catalysts and given the diversity of reactions amenable to photoredox processes, these novel artificial metalloenzymes will provide unique opportunities within selective catalysis.

To find out more see:

Light-driven electron injection from a biotinylated triarylamine donor to [Ru(diimine)₃]²⁺-labeled streptavidin
Sascha G. Keller, Andrea Pannwitz, Fabian Schwizer, Juliane Klehr, Oliver S. Wenger and Thomas R. Ward
DOI: 10.1039/C6OB01273F

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules, which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

The ubiquity of radioisotope-based bioprobes is being challenged by their nonradioactive counterparts. In recent years, they have gained significant popularity in life sciences due to major advances in available detection methods and enhanced analytical performance. Many would argue that radioactive labels offer superior results in experiments that require high sensitivity and resolution but their safe handling, stability and the proper disposal of radioactive materials limit speed and convenience of use.

The newest generation of fluorescent and chemifluorescent probes promise greater flexibility and versatility for a range of applications and the development of fully automated instrumentations and powerful imaging systems provide high throughput solutions to meet the increasing demands of the modern lab.

Unique photophysical properties can be incorporated into small biomolecules to generate fluorescent bioprobes. Fluorescently labeled nucleosides have distinct advantages over other synthetic molecules due to inherent fluorescence and minimal steric disruptions, and they can be tuned e.g. to form unusual base-pair preferences. They form noncovalent, highly specific duplexes with a complementary nucleic acid strand and are used to detect a defined DNA or RNA target sequence.

In a recent publication from the group of Yoshio Saito of Nihon University, the development of a novel nucleoside-based bioprobe containing a 3-deaza-2’-deoxyadenosine skeleton was reported. It behaves as an indicator for adenosine-cytosine base pair formation in oligodeoxynucleotide (ODN) duplexes by monitoring base-pair induced protonation. The probe displays distinct changes in its absorption and fluorescence activity as a result of its protonation state. In this way, the group is able to clearly discriminate cytosine from other bases on complementary strands based on absorption and fluorescence spectra.

The development of new biomarkers is providing insight to various genetic disorders, disease susceptibility, cancer predisposition and medication response. When fluorescent bioprobe imaging is coupled with genetic analytical techniques, such as single nucleoside polymorphism (SNP)-typing, the two synergize and provide a much more complete view than either one alone.

To find out more see:

Design and synthesis of a novel fluorescent benzo[g]imidazo[4,5-c]quinoline nucleoside for monitoring base-pair-induced protonation with cytosine: distinguishing cytosine via changes in the intensity and wavelength of fluorescence
Shogo Siraiwa, Azusa Suzuki, Ryuzi Katoh and Yoshio Saito
DOI:10.1039/C6OB00494F

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules, which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

In their search to solve complex biological problems by bridging gaps between protein synthesis and biological application, Prof. Jennifer Ottesen and her group at Ohio State University have been successfully developing what they term a ‘chemical toolbox’ for histone protein synthesis.

Within the field of epigenetics, heritable changes in gene expression outside of the DNA sequence are tightly regulated by post-translational modifications (PTMs) of DNA and histone proteins–the proteins that package DNA. Known PTMs of histone proteins include methylation, acetlyation, phosphorylation, sulfonylation and ubiquitination and fall under a hypothesized “histone code” which suggests that combinations of these markers alter DNA accessibility through chromatin restructuring and ultimately regulate gene expression.

In building synthetic histone proteins with distinct combinations of chemical modifications, the role of a specific sequence of PTMs in gene expression and the molecular mechanisms by which they function can be elucidated and targeted. This is of particular interest as epigenetics has become a hot topic in recent years due to an ever-growing understanding of these markers and their potential to act as selective entry points for disease intervention.

Ottesen’s recent publication in Organic and Biomolecular Chemistry outlines a new hybrid phase ligation approach for the synthesis of modified histone proteins which overcomes some long standing issues inherent in histone total synthesis. This method combines both solid and solution-phase ligation chemistry to improve process efficiency and overall yield. The group even demonstrates its ability to produce previously challenging CpA-K12ac histone protein which could not be synthesized with standard approaches.

Key to their success is the application of a dual-linker strategy which led to an efficient, sequence-independent resin attachment that liberates the desired native carboxy terminus of the protein which had been previously difficult to accomplish. Below is a scheme describing the solid-phase native chemical ligation of one of their desired targets, histone H4. A single coupling cycle includes deprotection followed by ligation and cleavage from the resin may be accomplished at either the Rink linker (black), or at the HMBA linker (red) to generate the native terminus.

Studies such as Prof. Ottesen’s are crucial as mechanisms by which certain genes are regulated must first be determined before developing targeted therapeutic approaches. Histone deacetylase (HDAC) inhibitors for example, interfere with histone deacetylase and have shown activity against various cancers, neurological diseases and immune disorders. The utility of this class of compound depends on their ability to target and modulate a subset of genes without causing global biological changes. Presently, additional work is required to define the human epigenome, its role in disease development and the processes that regulate it. Progress in the synthesis of highly desirable modified histone proteins brings us ever closer.

To find out more see:

Hybrid phase ligation for efficient synthesis of histone proteins
Ruixuan R. Yu, Santosh K. Mahto, Kurt Justus, Mallory M. Alexander, Cecil J. Howard, and Jennifer J. Ottesen
DOI: 10.1039/C5OB02195B

Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules, which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.